Kovrell: calls the vendor before you pay

Vendor bank-detail fraud starts with one email: "We changed banks, please pay the new account." 74% of US organizations were hit by business email compromise in 2025. 94% say they call the vendor back on a number from official records, but only 63% rate that control as very effective (AFP 2026). Callbacks get skipped under deadline pressure, get placed to the number in the fraudulent email, and accept "yes, that's us" as proof. Kovrell makes that callback automatic and hard to fool. When a bank-change request arrives, the payment is held. Kovrell checks the vendor's number of record and refuses to call if it changed in the last 30 days. Then a voice agent built on the AssemblyAI Voice Agent API calls the contact of record, confirms identity and the request, asks three questions drawn at random from the vendor's recent paid invoices, and reads back the new account. The model asks, and code decides. The agent never receives the answers, can't say whether one was right, and can't choose the verdict. Server code checks what it heard against the ledger and releases the payment, blocks it, or keeps it held. Every call is sealed into an evidence pack with the AssemblyAI recording and timeline, expected versus heard answers, and a sha256 hash, downloadable as JSON or PDF. The server holds the AssemblyAI socket, because the caller may be the fraudster: the vendor's browser is only an audio pipe. Kovrell uses per-run session.update, JSON-Schema tool calling, hold execution mode, max-accuracy transcription with keyterms, barge-in, and session artifacts. Try it live: set up your own vendor on /setup, take the call with your microphone, and watch a signed webhook deliver the verdict. 102 automated tests cover the attacks we tried.